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Charge transfer reaction of multi-charged oxygen ions with O2
Volume 86A, number 1
PHYSICS LETTERS
26 October 1981
CHARGE TRANSFER REACTION OF MULTI-CHARGED OXYGEN IONS WITH °2 H.M. HOLZSCHEITER and D.A. CHURCH Physics Department, Texas A & M University, College Station, TX 77843, USA
Received 12 June 1981
The rate of transfer of electrons from 02 to O~and been energies ~ 2compared eV using to a stored ion tech9cm3/s andO~ for has O~1, k = measured 2.5(0.3) xatj~—9 cm3/s, calculated nique. The rate for O~ is k = 1.0(0.3) X i0~ Langevin rates of 1.8 x i0~cm3/s and 2.7 X i09 cm3/s respectively.
The charge dependence of atomic physics interactions offers an additional dimension to both spectroscopic and collision studies, and provides a useful vanable with which to scale those interactions. There is currently substantial theoretical and experimental activity directed toward electron transfer to low-energy multiply charged ions, motivated by fusion research, which includes ion—molecule processes [1], and by astrophysical significance [2]. At relative energies <~5 eV, the most rapid reactions often occur at the Langevin rate independent of energy, implying a cross section inversely proportional to velocity. At these lower relative energies, less probable electron transfer interactions will have an energy-dependent rate. Several calculations for ions in various charge states colliding with a wide range of predicted rates at near-thermal energies with hydrogen and helium atoms [3,4] have recently been completed. Preparatory to the study of low-energy multicharged ion collisions with hydrogen atoms, we have measured the reaction rates for charge transfer from 02 to 02+ and 0~ ions in an initial demonstration of the higher charge state proceeding capabilities at possible for vacuum charge transfer measurements ultrahigh pressures with low energy magnetically confined ions [5,6]. This is thought to be the first measurement of electron transfer from a molecule to an ion with charge greater than two units at such low relative energies, although transfer from a Ne atom to a highly charged ion at slightly higher relative energy has been studied recently [7].
The ions were produced by electron impact ionization of gas with partial pressure as low as 2 X 10—10 Torr inside a Penning-type ion trap [8]. Following storage for a determined interval T~,the ion number was measured by a resonant absorption method. This cycle is repeated for a range of intervals 7, the reactant gas partial pressure flg is determined, and the measured time constant r for the exponential decay of ion number gives a reaction rate k = (ngrY~. For production of highly charged ions, for partial pressure measurements, and for interpretation of data, the harmonic motion of the ions stored in the Penning trap is a decisively important feature for measurements of this type [5,6]. The ions move axially with an angular oscillation frequency wz = (qV/mz~)~”2 determined by a quadrupole dc potential V and the ion charge-to-mass ratio q/m, in a trap with axial electrode separation 2z 0 1 cm. Radially, the defocussing action of the quadrupole potential is compensated by a uniform axial magnetic field B 1 T to produce a shifted cyclotron angular frequency w~.A magnetron oscillation of the ions around the symmetry axis at 3z~) depends weakly w_ = V0/2Bz~ + (m/q)( V~/4B on m/q. Radio-frequency resonant excitation of Unwanted q/m ratios at any of these frequencies particularly during the ionizing electron pulse, permits the isolation and accumulation of a desired q/m ratio. The principal benefits arising from this procedure are twofold. Firstly, since electron impact ionization is nonselective, and cross sections decrease with increasing q, lower charged ions would rapidly fill the trap to the 25
Volume 86A, number 1
PHYSICS LETTERS
space-charge limit unless eliminated soon after formation: this selective removal permits confinement of relatively large numbers of the desired q/m. Secondly, since multiple electron transfer is expected to be nonnegligible, and in some cases dominant [6] the storage of a range of q/m ratios would permit accurate measurements for only the highest q/m ratio due to simultaneous formation and loss of stored ions at lower charge states, making analysis difficult. Otherwise small numbers of other ions have not been observed to affect the measurements. The 03+ ions were produced by electron impact ionization primarily of stored 02+ ions, which were held during the ionization interval, while ions with m/q ~ 4 were removed by swept cyclotron excitation, and ions with m/q> 14 were eliminated by swept w_ excitation. The ionizing electrons were introduced along the magnetic field lines. Resonant elimination of 02+ during the electron pulse reduced the ~ signal by more than 50% at 200 eV electron energy, indicating that single ionization of 02+ is important. Although these initial ionization measurements have not been fully explored, the potential for the study of electron impact ionization of ions by related means is noteworthy. Once sufficient 03+ ions were stored, their number was detected by sweeping the w~oscillation frequency through the resonance of an excited tuned circuit with Q = 140 composed of the trap capacitance and an external inductance. The envelope of the exciting rf is reduced in proportion to the number of ions at each m/q. Fig. la shows an example of the spectrum containingm/q values from 5.33 (03÷)to 14 (N~)protively identified as Ar4~.The resolution of the spectrum is limited by the Q of the tuned circuit, so m/q identification is made by selectively removing each peak by cyclotron resonance. Fig. lb shows the mass duced by this method. The m/q = 10 peak is tentaspectrum trapped during the 02+ measurements. The improvement in resolution comes from increasing the tuned circuit Q to 220. The signals in fig. lb are about 8 times larger than those in fig. Ia. Fig. 2 shows the 03+ number versus storage time T 5 at several different 02 partial pressures. Total pressures were obtained with a nude ion gauge calibrated in the 1O~—l0~Torr range against a capacitance manometer. Relative partial pressures were measured with the Penning trap operated in an ion-gauge mode 26
26 October 1981 I
(0)
2’) N~ —
C
I I
4
z
0~ (b)
Z 0 —
N~
..
0
5 10 15 2Otamu/ql MASS-TO-CHARGE RATIO
Fig.+ 1. (a) Ion number of m/q values during measurements; (b) spectrum similar spectrum for 02trapped + measurements, with improved resolution.
[6] by detecting all ions produced by 70 eV electrons at low enough currents to avoid space-charge saturation of any signal. The data were taken at partial pressures of 02 varied in a range from 9.9 X iO—9 to 1.5 X l0—~ Torr. The 03+ data were least-squares fit to a single exponential and the 02+ data separately fit to one and to two exponentials, to test for possible effects due to
—
—
31 02 Den i~~io9 cm31
z
35 .1
.2
.3 .I~ .5 [sec] STORAGE TIME Fig. 2. Normalized number of ~ ions versus storage time, at four 02 densities.
Volume 86A, number 1
PHYSICS LETTERS
the long-lived 1D and i~metastable levels, which might have different reaction rates. The best fits were obtamed to the single exponential, indicating that metastables do not play a significant role in determining the rate constant. The results are: k(O3~,02) 2.5(0.3) X l0—~cm3/s and k(O2~,02) = 1.0(0.3) X l0~ cm3/s. The estimated mean ion temperatures are T(O2~)= 8(3) X 10~K and T(O3~)= 12(3) X 10~K. Flow-drift tube measurements [10 11] give k(O2~0,) = 1.5(0.3) X 1 0~ cm3 near 400 K, which is within the statistical error of our higher temperature measurements. The effective axial well depth of the trap was additionally increased by 30% for some of the 02+ measurements to test for a possible energy dependence of the reaction rate. Ions are typically stored with mean energies 10% of the axial well depth [9], and linear mean energy dependences on well.depth are observed for 02+ and 03+. Fig. 3 shows the 02+ signal versus storage time at the 10 V and 15 V well depths used, measured at the same 02 partial pressure. It is seen that the measurements are independent of well depth, i.e. mean ion energy. The lack of an observed dependence of the rate constant on this parameter, together with the results quoted above, demonstrate that the rate constant is essentially energy independent in the range of energies studied. This permits the inference that the 02+ cross section for charge transfer has a ~r’ velocity dependence of the Langevin type [12], despite a reaction rate lower than the Langevin value kL = 1.8 X l0~ cm3/s, and in contrast to measurements at higher relative energy on other ions [7]. The observation of products is necessary to definitively identify the number of electrons transferred in a collision. Product ions are sometimes not observed /5
-~
.
I
1
STORAGE Fig. 3. Decay of02
+
2 TIME
number with time measured at two axial 3. an 02 density flg =
7 x
,-~
~
+
‘-‘
-~ U
U
,-.~ U
.i
~~-‘-
~
-~
This research was initiated with funds from the Texas A & M Center for Energy and Mineral Resources, and was supported by the Department of Energy,
(secl
well depthsD = V 0/2, and
in higher-charged ion reactions, due to the Coulomb explosion of the intermediate complex following charge transfer: this is the case in these measurements. The fast products are lost rapidly from the trap apparently during the detection sweep of the quadrupole potential, when the well depth is considerably reduced. The most likely transfer reactions of 03+ in this collision are: ,-~2+ ~-2+ + ‘-‘2 + “-‘2) + + + e + 02 -÷ O~+ (o~+)* -÷ 0~+ 0~+ 0+ + 9 eV. (2) Since the sum of ionization plus dissociation energies of 02 makes the single charge transfer process (1) exothermic by only about ~E 1 eV, the relatively low energy products should be observed if produced. However, consideration of the mutual repulsion of the 02+ and 0~products indicates that the transfer must occur at an unreasonably large separation r 2e2/~E 25 A. The energetics of the double charge transfer process (2) indicate that can be formed in a vibrationally excited level of the weakly bonding A3~state [13] leading to a subsequent dissociation with a mean energy of about 3 eV per 0~ion, apparently leading to undetected ion loss. For 02+ the situation is less clear. The reaction 02+ + 02 -÷ 0 + 0~(X1~) is exothermic by 2.8 eV and hence cannot be rejected. The normally stable 0~ ground state may dissociate into fast products due to curve crossings upon separation of the collision products. The single charge transfer process 02+ + 02 0~+ 0~+ 0 + 9.3 eV is also exothermic with sufficient energy to permit transfer at reasonable distances. We have measured the charge transfer rates for collisions of oxygen ions having q 3 with 02, at relative energies ~2 eV. We find that for q = 2 and 3, the cross sections appear to have a v~ dependence. Definitive identification of the number of charges transferred requires the containment and identification of the collision products. Efforts to achieve this, and to extechnique to still higher charge states, are in
V - 30 Vott V~ 20 volt:
~
26 October 1981
108 cm
Division of Chemical Sciences, under contract DE-
ASO5-78ER6043. We thank Professor A.L. Ford for helpful comments. 27
Volume 86A, number 1
PHYSICS LETTERS
References [1] H.W. Drawin, J. de Phys. Cl (1979) 73. [21 A. Dalgarno and S.E. Butler, Comments At. Mol. Phys. 7 (1978) 129. [3] S.E. Butler, A. Dalgarno and CF. Bender, Astrophys. ~ Lett. 230 (1979) 159; S.E. Butler, T.G. Hell and A. Dalgarno, Astrophys. J. 241 (1980) 442. [4] J.S. Cohen and J.N. Bardsley, Phys. Rev. A18 (1978) 1004. [5] D.A. Church and H.M. Holzcheiter, Chem. Phys. Lett. 76 (1980) 109. [6] H.M. Holzscheiter and D.A. Church, J. Chem. Phys. 74 (1981) 2313.
28
26 October 1981
17]
C.R. Vane, M.H. Prior and R. Marrus, Phys. Rev. Lett. 46 (1981) 107.
18]
H.G. Dehmelt, Adv. At. Mol. Phys. 3 (1967) 53; 5 (1969) 109. R.A. Heppner, F.L. Walls and G.H. Dunn, Phys. Rev. Al 3 (1976) 1000. R. Johnsen and M.A. Biondi, Geophys. Res. Lett. 5 (1978) 847. F. Howorka, A.A. Viggiano, D.L. Albritton,E.E. Ferguson, and F.C. Fehsenfeld, J. Geophys. Res. 84, (1979) 5941. See, e.g. M.T. Bowers and T. Su, Adv. Electron. Electron Phys.,Vol. 34, ed. L. Martin (Academic Press, New York, 1973) ii. 223. A.C. Huxley, J. Mol. Spectrosc. 9 (1962) 18.
19] [101 [11]
[12]
[13]
PHYSICS LETTERS
26 October 1981
CHARGE TRANSFER REACTION OF MULTI-CHARGED OXYGEN IONS WITH °2 H.M. HOLZSCHEITER and D.A. CHURCH Physics Department, Texas A & M University, College Station, TX 77843, USA
Received 12 June 1981
The rate of transfer of electrons from 02 to O~and been energies ~ 2compared eV using to a stored ion tech9cm3/s andO~ for has O~1, k = measured 2.5(0.3) xatj~—9 cm3/s, calculated nique. The rate for O~ is k = 1.0(0.3) X i0~ Langevin rates of 1.8 x i0~cm3/s and 2.7 X i09 cm3/s respectively.
The charge dependence of atomic physics interactions offers an additional dimension to both spectroscopic and collision studies, and provides a useful vanable with which to scale those interactions. There is currently substantial theoretical and experimental activity directed toward electron transfer to low-energy multiply charged ions, motivated by fusion research, which includes ion—molecule processes [1], and by astrophysical significance [2]. At relative energies <~5 eV, the most rapid reactions often occur at the Langevin rate independent of energy, implying a cross section inversely proportional to velocity. At these lower relative energies, less probable electron transfer interactions will have an energy-dependent rate. Several calculations for ions in various charge states colliding with a wide range of predicted rates at near-thermal energies with hydrogen and helium atoms [3,4] have recently been completed. Preparatory to the study of low-energy multicharged ion collisions with hydrogen atoms, we have measured the reaction rates for charge transfer from 02 to 02+ and 0~ ions in an initial demonstration of the higher charge state proceeding capabilities at possible for vacuum charge transfer measurements ultrahigh pressures with low energy magnetically confined ions [5,6]. This is thought to be the first measurement of electron transfer from a molecule to an ion with charge greater than two units at such low relative energies, although transfer from a Ne atom to a highly charged ion at slightly higher relative energy has been studied recently [7].
The ions were produced by electron impact ionization of gas with partial pressure as low as 2 X 10—10 Torr inside a Penning-type ion trap [8]. Following storage for a determined interval T~,the ion number was measured by a resonant absorption method. This cycle is repeated for a range of intervals 7, the reactant gas partial pressure flg is determined, and the measured time constant r for the exponential decay of ion number gives a reaction rate k = (ngrY~. For production of highly charged ions, for partial pressure measurements, and for interpretation of data, the harmonic motion of the ions stored in the Penning trap is a decisively important feature for measurements of this type [5,6]. The ions move axially with an angular oscillation frequency wz = (qV/mz~)~”2 determined by a quadrupole dc potential V and the ion charge-to-mass ratio q/m, in a trap with axial electrode separation 2z 0 1 cm. Radially, the defocussing action of the quadrupole potential is compensated by a uniform axial magnetic field B 1 T to produce a shifted cyclotron angular frequency w~.A magnetron oscillation of the ions around the symmetry axis at 3z~) depends weakly w_ = V0/2Bz~ + (m/q)( V~/4B on m/q. Radio-frequency resonant excitation of Unwanted q/m ratios at any of these frequencies particularly during the ionizing electron pulse, permits the isolation and accumulation of a desired q/m ratio. The principal benefits arising from this procedure are twofold. Firstly, since electron impact ionization is nonselective, and cross sections decrease with increasing q, lower charged ions would rapidly fill the trap to the 25
Volume 86A, number 1
PHYSICS LETTERS
space-charge limit unless eliminated soon after formation: this selective removal permits confinement of relatively large numbers of the desired q/m. Secondly, since multiple electron transfer is expected to be nonnegligible, and in some cases dominant [6] the storage of a range of q/m ratios would permit accurate measurements for only the highest q/m ratio due to simultaneous formation and loss of stored ions at lower charge states, making analysis difficult. Otherwise small numbers of other ions have not been observed to affect the measurements. The 03+ ions were produced by electron impact ionization primarily of stored 02+ ions, which were held during the ionization interval, while ions with m/q ~ 4 were removed by swept cyclotron excitation, and ions with m/q> 14 were eliminated by swept w_ excitation. The ionizing electrons were introduced along the magnetic field lines. Resonant elimination of 02+ during the electron pulse reduced the ~ signal by more than 50% at 200 eV electron energy, indicating that single ionization of 02+ is important. Although these initial ionization measurements have not been fully explored, the potential for the study of electron impact ionization of ions by related means is noteworthy. Once sufficient 03+ ions were stored, their number was detected by sweeping the w~oscillation frequency through the resonance of an excited tuned circuit with Q = 140 composed of the trap capacitance and an external inductance. The envelope of the exciting rf is reduced in proportion to the number of ions at each m/q. Fig. la shows an example of the spectrum containingm/q values from 5.33 (03÷)to 14 (N~)protively identified as Ar4~.The resolution of the spectrum is limited by the Q of the tuned circuit, so m/q identification is made by selectively removing each peak by cyclotron resonance. Fig. lb shows the mass duced by this method. The m/q = 10 peak is tentaspectrum trapped during the 02+ measurements. The improvement in resolution comes from increasing the tuned circuit Q to 220. The signals in fig. lb are about 8 times larger than those in fig. Ia. Fig. 2 shows the 03+ number versus storage time T 5 at several different 02 partial pressures. Total pressures were obtained with a nude ion gauge calibrated in the 1O~—l0~Torr range against a capacitance manometer. Relative partial pressures were measured with the Penning trap operated in an ion-gauge mode 26
26 October 1981 I
(0)
2’) N~ —
C
I I
4
z
0~ (b)
Z 0 —
N~
..
0
5 10 15 2Otamu/ql MASS-TO-CHARGE RATIO
Fig.+ 1. (a) Ion number of m/q values during measurements; (b) spectrum similar spectrum for 02trapped + measurements, with improved resolution.
[6] by detecting all ions produced by 70 eV electrons at low enough currents to avoid space-charge saturation of any signal. The data were taken at partial pressures of 02 varied in a range from 9.9 X iO—9 to 1.5 X l0—~ Torr. The 03+ data were least-squares fit to a single exponential and the 02+ data separately fit to one and to two exponentials, to test for possible effects due to
—
—
31 02 Den i~~io9 cm31
z
35 .1
.2
.3 .I~ .5 [sec] STORAGE TIME Fig. 2. Normalized number of ~ ions versus storage time, at four 02 densities.
Volume 86A, number 1
PHYSICS LETTERS
the long-lived 1D and i~metastable levels, which might have different reaction rates. The best fits were obtamed to the single exponential, indicating that metastables do not play a significant role in determining the rate constant. The results are: k(O3~,02) 2.5(0.3) X l0—~cm3/s and k(O2~,02) = 1.0(0.3) X l0~ cm3/s. The estimated mean ion temperatures are T(O2~)= 8(3) X 10~K and T(O3~)= 12(3) X 10~K. Flow-drift tube measurements [10 11] give k(O2~0,) = 1.5(0.3) X 1 0~ cm3 near 400 K, which is within the statistical error of our higher temperature measurements. The effective axial well depth of the trap was additionally increased by 30% for some of the 02+ measurements to test for a possible energy dependence of the reaction rate. Ions are typically stored with mean energies 10% of the axial well depth [9], and linear mean energy dependences on well.depth are observed for 02+ and 03+. Fig. 3 shows the 02+ signal versus storage time at the 10 V and 15 V well depths used, measured at the same 02 partial pressure. It is seen that the measurements are independent of well depth, i.e. mean ion energy. The lack of an observed dependence of the rate constant on this parameter, together with the results quoted above, demonstrate that the rate constant is essentially energy independent in the range of energies studied. This permits the inference that the 02+ cross section for charge transfer has a ~r’ velocity dependence of the Langevin type [12], despite a reaction rate lower than the Langevin value kL = 1.8 X l0~ cm3/s, and in contrast to measurements at higher relative energy on other ions [7]. The observation of products is necessary to definitively identify the number of electrons transferred in a collision. Product ions are sometimes not observed /5
-~
.
I
1
STORAGE Fig. 3. Decay of02
+
2 TIME
number with time measured at two axial 3. an 02 density flg =
7 x
,-~
~
+
‘-‘
-~ U
U
,-.~ U
.i
~~-‘-
~
-~
This research was initiated with funds from the Texas A & M Center for Energy and Mineral Resources, and was supported by the Department of Energy,
(secl
well depthsD = V 0/2, and
in higher-charged ion reactions, due to the Coulomb explosion of the intermediate complex following charge transfer: this is the case in these measurements. The fast products are lost rapidly from the trap apparently during the detection sweep of the quadrupole potential, when the well depth is considerably reduced. The most likely transfer reactions of 03+ in this collision are: ,-~2+ ~-2+ + ‘-‘2 + “-‘2) + + + e + 02 -÷ O~+ (o~+)* -÷ 0~+ 0~+ 0+ + 9 eV. (2) Since the sum of ionization plus dissociation energies of 02 makes the single charge transfer process (1) exothermic by only about ~E 1 eV, the relatively low energy products should be observed if produced. However, consideration of the mutual repulsion of the 02+ and 0~products indicates that the transfer must occur at an unreasonably large separation r 2e2/~E 25 A. The energetics of the double charge transfer process (2) indicate that can be formed in a vibrationally excited level of the weakly bonding A3~state [13] leading to a subsequent dissociation with a mean energy of about 3 eV per 0~ion, apparently leading to undetected ion loss. For 02+ the situation is less clear. The reaction 02+ + 02 -÷ 0 + 0~(X1~) is exothermic by 2.8 eV and hence cannot be rejected. The normally stable 0~ ground state may dissociate into fast products due to curve crossings upon separation of the collision products. The single charge transfer process 02+ + 02 0~+ 0~+ 0 + 9.3 eV is also exothermic with sufficient energy to permit transfer at reasonable distances. We have measured the charge transfer rates for collisions of oxygen ions having q 3 with 02, at relative energies ~2 eV. We find that for q = 2 and 3, the cross sections appear to have a v~ dependence. Definitive identification of the number of charges transferred requires the containment and identification of the collision products. Efforts to achieve this, and to extechnique to still higher charge states, are in
V - 30 Vott V~ 20 volt:
~
26 October 1981
108 cm
Division of Chemical Sciences, under contract DE-
ASO5-78ER6043. We thank Professor A.L. Ford for helpful comments. 27
Volume 86A, number 1
PHYSICS LETTERS
References [1] H.W. Drawin, J. de Phys. Cl (1979) 73. [21 A. Dalgarno and S.E. Butler, Comments At. Mol. Phys. 7 (1978) 129. [3] S.E. Butler, A. Dalgarno and CF. Bender, Astrophys. ~ Lett. 230 (1979) 159; S.E. Butler, T.G. Hell and A. Dalgarno, Astrophys. J. 241 (1980) 442. [4] J.S. Cohen and J.N. Bardsley, Phys. Rev. A18 (1978) 1004. [5] D.A. Church and H.M. Holzcheiter, Chem. Phys. Lett. 76 (1980) 109. [6] H.M. Holzscheiter and D.A. Church, J. Chem. Phys. 74 (1981) 2313.
28
26 October 1981
17]
C.R. Vane, M.H. Prior and R. Marrus, Phys. Rev. Lett. 46 (1981) 107.
18]
H.G. Dehmelt, Adv. At. Mol. Phys. 3 (1967) 53; 5 (1969) 109. R.A. Heppner, F.L. Walls and G.H. Dunn, Phys. Rev. Al 3 (1976) 1000. R. Johnsen and M.A. Biondi, Geophys. Res. Lett. 5 (1978) 847. F. Howorka, A.A. Viggiano, D.L. Albritton,E.E. Ferguson, and F.C. Fehsenfeld, J. Geophys. Res. 84, (1979) 5941. See, e.g. M.T. Bowers and T. Su, Adv. Electron. Electron Phys.,Vol. 34, ed. L. Martin (Academic Press, New York, 1973) ii. 223. A.C. Huxley, J. Mol. Spectrosc. 9 (1962) 18.
19] [101 [11]
[12]
[13]